iio: adc: ad7298: make use of iio_get_masklength()

Use iio_get_masklength() to access '.masklength' so it can be annotated
as __private when there are no more direct users of it.
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240702-dev-iio-masklength-private-v1-15-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7298.c

@@ -109,7 +109,8 @@ static int ad7298_update_scan_mode(struct iio_dev *indio_dev,
 	int scan_count;
 
 	/* Now compute overall size */
-	scan_count = bitmap_weight(active_scan_mask, indio_dev->masklength);
+	scan_count = bitmap_weight(active_scan_mask,
+				   iio_get_masklength(indio_dev));
 
 	command = AD7298_WRITE | st->ext_ref;